Indium is a comparatively rare metal with an abundance in the earth's crust of approximately 1.0 ppm. Its major use is as an alloying element in the manufacture of heavy duty bearings, soldering compounds and electrical fuses. Electronics applications in the preparation of indium phosphide semiconductors and optical devices are gaining in importance.
The most important source of indium is as a by-product from primary zinc and zinc-lead production. Processing involves the recovery of indium by solvent extraction with di-2-ethylhexyl phosphoric acid (D2EHPA) from acidic sulphate leach liquors derived, for example, from flue dusts. However, indium, like ferric iron, is known from Corinea, et al., U.S. Pat. No. 3,462,352, to be strongly extracted by D2EHPA and excessive quantities of strong mineral acids, e.g., 8-9M HCl are needed for back-extraction or stripping. Handling such volumes of acid on an industrial scale poses difficult problems in terms of safety, waste disposal and corrosion.
In Rickelton, U.S. Pat. No. 4,778,663, is disclosed a process for the recovery of uranium from wet process phosphoric acid. It is reported therein that the use of a combination of a dialkyl phosphoric acid and a mixture of at least four trialkyl phosphine oxides substantially enhances the extraction of uranium from wet process phosphoric acid because of phase modification by the phosphine oxides, i.e., the uranium forms a water insoluble complex with the phosphine oxide.
In Rickleton and Boyle, U.S. Pat. No. 5,028,334, a process is disclosed to selectively recover tin from aqueous acid solutions using extractant compositions comprising branched phosphine oxide compounds, alone, or in combination with mixtures of at least four phosphine oxides which appear to form water insoluble complexes with tin.
In neither citation, however, is there any hint or suggestion that modifying the D2EHPA with the specific phosphine oxides will lead to essentially quantitative indium stripping with weaker acids, thus reducing waste disposal and corrosion problems.
Thus, a need still exists for a more efficient process to recover the relatively small amounts of valuable indium from the acidic sulphate leach liquors.
It has now been unexpectedly discovered that enhanced stripping of indium can be obtained with a solvent extractant combination of (i) a dialkyl phosphoric acid, e.g. DEHPA, and (ii) 1) a mixture of at least four phosphine oxides containing unsymmetrical tertiary phosphine oxides, 2) a mixture of branched tertiary phosphine oxides, or 3) a combination of 1) and 2). The phosphine oxide mixture 1) containing at least four components is prepared by the reaction of phosphine and a mixture of two olefins followed by oxidation of the resulting trialkyl phosphine mixture to the corresponding tertiary phosphine oxides. The amount of each component is carefully controlled by the ratio of the two olefins reacted with phosphine in the range of 10:90 to about 90:10 and preferably, in the range of 60:40 to 40:60. The phosphine oxide mixture 2) containing the branched compounds is prepared also by the reaction of phosphine and a mixture of two olefins, but in this instance, sequentially, one of the olefins is branched, and the other unbranched. The instant process is advantageous in that it provides substantially higher indium stripping efficiency than the conventional DEHPA extraction solvent mixture, thus reducing the number of stripping stages and permitting the use of relatively weak acids, e,g., 2M.H.sub.2 SO.sub.4. An added advantage is that the presence of the phosphine oxides prevents the accumulation of a difficult-to-strip ferric iron complex from iron in the leach liquors into the solvent. This complex, see, Rickelton, France Demande, FR 2,481,941, corresponding to Belgium Patent 888,708, Nov. 9, 1981, is believed to be an Fe.sup.+++ -D2EHPA polymer having a molecular weight of approximately 100,000. Its presence can result in high viscosities, consequently poor phase disengagement rates and, eventually, solvent poisoning.
The tertiary phosphine oxide mixture used in the process of this invention is disclosed in U.S. Pat. No. 4,909,939, granted Mar. 20, 1990. This mixture is reported to extract acetic acid and phenol from aqueous solutions. The branched tertiary phosphine oxides used in the process of this invention are disclosed on U.S. Pat. No. 5,028,334 in connection with the tin recovery process mentioned above.